Thermionic amplifying system



3 Sheets-Sheet 1 Filed March 21, 1932 l vi INVENTOR e. A. MAZIEU WW ATToRNiY July 31, 1934.

THERMIONI C AMPLIFYING S YSTEM Filed March 21 1932 3 Sheets-Sheet 2 BY l m A.TTC')RNEY e A. MATHlEU 1,968,610

Patented July 31, 1934 PATENT OFFlC THERMIONIC AMPLIFYING SYSTEM Gaston Adelin Mathieu, London, England, assigner to Radio Corporation of America, a corporation of Delaware Application March 21,

1932, Serial No. 600,104

In Great Britain March 23, 1931 11 Claims.

The invention relates to thermionic amplifying systems andmore particularly to a thermionic receiving amplifier of high sensitivity suitable for use on very short waves.

l'he invention provides an improved amplifying system of the kind-in which a frequency changer is applied for changing the frequency of the signalsto be amplified.

lhe invention is illustrated by and explained in connection with the accompanying drawings in which Figure 1 shows a known frequency changer arrangement; Figure 2 a frequency changing arrangement of the general type illustrated in Figure lbut improved in accordance with the present invention, Figure 3 a modification of the arrangement shown in Figure 2, and Figures 4 and 5 are constructions of a condenser suitablefor use in carrying out the inventions. Figures 6 and 7 illustrate, in section, condenser arrangements which may be utilized in this invention. Figure 8 is a preferred schematic arrangement for obtaining a weak coupling effect. Figure 9 is a front view'of a portion of the apparatus used in Figure 8. Figure 10 illustrates a preferred arrangement for coupling the aerial to the first stage of a high frequency amplifier of a receiver.

Referring to Figure 1,-this shows a known arrangement which has been employed as a frequency changer in an amplifying system for amplifying signals; As will be seen this frequency changing arrangement consists of a pair of valves whose grids are connected to one another through a pair of condensers in series and also through an inductance in shunt across said condensers, the middle point between the two condensers being earthed and the mid tapping on the coil shunting said condensers being taken through a further coil to earth, said further coil receiving local oscillations impressed through a circuit indicated in the figfrom the signal input circuit SI via the coupling C. Now when'the circuit shown in Figure 1 ispreceded by a powerful high frequency amplifier, and,

l more especially, whenit is employed for receiving shortwavesof theorder of lo metres or less, se-

rious practical difficulties are introduced by reason of the very high noise level which occurs, in part by reason of the liability of the detector valves to break into self-oscillation and in part owing to the difficulty of obtaining an effective and satisfactory coupling to the output circuit of the high frequency amplifier. It will be obvious that it is desirable that there be a very strong action from the local oscillator on the two grid circuits of the push pull modulating valves which are working in parallel. In the circuit shown in the figure, the local oscillator voltages are applied from the middle of the grid coil, and it is quite difficult to obtain a strong action in this way, es pecially at very high frequencies, owing to the choking effect of the grid coil and also owing to the low reactances offered between the grids and earth bythe tuning condensers. Moreover, unless the tapping on the grid coil be exactly at the centre (a requirement which in practice is hardly ever realized) circulating current due to the local oscillator will fiow round the valve input circuit and thus the symmetry of the arrangement and many of the advantages of the circuit would be lost. Further it is exceedingly difiicult to obtain two valves whose characteristics are and will. remain exactly the same. Another serious disadvantage arises by reason of the tendency of the modulating valves to oscillate, due to the starting action of the local oscillator when it comes in tune with the output circuit of the frequency changer during tuning operations.

The present invention provides an improved circuit arrangement in which these disadvantages are avoided.

The invention also provides certain detailed improvements in arrangements for applying amplified radio frequency signals to the frequency changer of a superheterodyne receiver in accordance with the principal feature of the invention.

According to this principal feature of this invention, a frequency changer. arrangement suitable for use in connection with a receiver for receiving very short wave lengths comprises two modulating valves whose grids are electrostaticcircuit arrangement of Figure 1, modified in accordance with the present invention. As will be seen, local oscillations from a source LO are applied through a condenser C13 and through two variable condensers (3108 to the grids of the modulating valves V1V2, these grids being also coupled through variable condensers CsCi to the signal input circuit generally indicated at SI. R132 are a pair of as nearly as possible pure ohmic resistances in series with batteries E31, E32, for applying grid voltage to the valves V1V2. If desired, potentiometers may be associated with the batteries E81 and E152 and in certain cases the resistances R132 may be replaced by chokes or chokes in series with resistances. The output circuit includes condensers C90 0 and chokes CH3 and CH1 for the purpose of preventing any possibility of plate modulation. These chokes and condensers though desirable are not indispensable. Jacks J1 and J2 are provided in shunt with condensers C11 and C12 to facilitate checking up the various circuit adjustments and the output circuit inductances are shown at L1 and L1. Where the arrangement is for use on very short wave lengths, the provision of condensers C5 and Cs and filament chokes CH1 and CH2, as shown, will be found to be of advantage.

The condenser C13 serves to prevent any inductive coupling between the local oscillator and the valves. This condenser may conveniently be constructed as a condenser tube with air or any other suitable dielectric, and may form constructionally part of the circuit. For example, the arrangement shown in Figures 4 and 5 of the drawings may be conveniently employed, Figure 4 being a schematic sectional elevation and Figure 5 being an end view. The condenser C13 may, however, be dispensed with altogether, though its presence will generally be found of advantage.

Figure 3 of the drawings shows a modification of the arrangement shown in Figure 2, and it will be noted that high frequency chokes CH4 and CH5 have been introduced to prevent circulating current from the local oscillator flowing through the fairly large capacities C3 and C4, and through the secondary windings of the input transformer TR. C14 and C15 are so called neutralizing condensers. If desired, the valves V1 and V2 may be replaced by so -called screened grid valves, as also can the valves V1 and V2 of Figure 2. Where it is desired to cause the output circuit to admit the two side bands, resistances R3 and R4 may be introduced, the output circuit being tuned to the same frequency as that of the local oscillator. In the case of one side band transmission only. however, the resistances R3 and R1. should be omitted and the output circuit made of as low damping as possible and tuned to one of the sideband frequencies.

Preferably the condensers C1Ca of Figures 2 and 3 of the drawings are constituted by a single condenser structure which may be, and preferably is, a differential condenser structure.

Figure 5 of the accompanying drawings shows, in schematic section, a form of differential condenser which may be employed to constitute the said condensers C1 and C3, the figure showing, also in diagrammatic form, just suificient of the circuit of Figure 2 or Figure 3 to identify the connections of the said condenser structure.

Referring to the accompanying Figure 6 in more detail, the condenser structure comprises an insulated carrier member HC'ZS upon which is mounted a pair of fixed tubular electrodes AC1 and AC8, these electrodes corresponding to those electrodes of the condensers C1 and Cs which are directly connected to the grids of the valves V1 and V2. Electrostatically coupled to the electrodes AC1 and ACa is a movable electrode or condenser armature A078 which, as shown, is so mounted that it may, by operation of a handle I-IC1s (which actuates a screw), be moved towards the fixed electrode AC1 or towards the fixed electrode AC8. The armature A078 is electrically connected to the condenser C1; of Figures 2 and 3 and thence to a coil which is coupled to the local oscillator LO. It will be seen that the condenser illustrated in Figure 6 is a differential condenser since any movement of the handle H018 to increase the capacity of the condenser C1 will also decrease the capacity of the condenser C8, and vice versa.

Figure '7 of the accompanying drawings shows another arrangement which may be employed in place of that shown in Figure 6. The construction. shown in Figure 7 comprises, in effect, two independently separately adjustable condensers, one of which constitutes the condenser C1, and the other the condenser 08. These condensers have a common electrode AC'vs which is tubular and of the form shown in section in Figure 2 of the drawings. The electrode AC'1s is mounted upon an insulating bar B by means of a set screw SC78 which passes through said electrode and bar as illustrated. On the bar B is adjustably mounted a pair of tubular movable electrodes AC1 and AC':; which electrodes are so dimensioned that they may be entered into the electrode ACvsat either end thereof to adjustable extents. The electrodes AC'1 and AC's are held in any predetermined position of adjustment by screws SCAG: and SCAC's which pass through the said electrodes into the central bar B. The connections to the condenser structure shown in the accompanying Figure 7 correspond to those of the structure shown in Figure 6.

A superheterodyne receiver in accordance with this invention may, of course, and in general practice ordinarily will, include a high frequenn in accordance with the present invention, if the heterodyne wave impressed on the grids of the input circuit to the frequency Figure 8 of the accompanying drawings shows diagrammatically a preferred arrangement whereby a weak adjustable coupling may be obtained. In this figure, V3V1 represent the valves of a final push-pull connected stage of a radio frequency amplifier, the anode potential being applied to the valves via a centre tapping upon a choke HFCh. The stage including the valves VaV4 is enclosed within a screening box S131 and the frequency changer stage within a screening box SE2, this box having a common partition PSB. The partition PSB is provided with holes or apertures and on either side of these holes or apertures are arranged condenser electrodes which, as indicated in the accompanying Figure 8, constitute the condensers coupling the valves Va /i, to the circuit SI. The coupling condensers thus constituted are indicated by the reference letters C811 and C312 and are preferably so arranged that there is no need to move their constituent electrodes to vary their capacities, the desired adjustment being obtained by cuttingoff to an adjustable degree the apertures in thepartition PSB. Figure 9 of the accompanying drawings shows schematically one arrangement whereby the variation of the capacity of the condensers C511 and C312 may be effected. Referring to Figure 9, the apertures in the partition PSB are indicated at E0311 and HCSIz. Rotatably mounted upon the partition PSB is a circular disc CD said disc being pivoted at PCD and being arranged to be rotated for example, by a worm drive from a handle (not shown) which is attached to the shaft AH. The circular disc CD is provided with two apertures I-ICSI1 and IZCSIz of the same size as the apertures in the partition and so positioned that as the disc CD is rotated the apertures in the partition may be masked to a greater or less extent, the masking of course being determined by the position of re circular disc and the apertures therein with respect to the positions of the fixed apertures in the partition. Thus by rotating the shaft AH,

, the value of the coupling condensers CSI1 and CEiIz may be varied.

It is, of course, to be understood that one electrode of each of the condensers C811 and CSIz is arranged on one side of the appropriate aperture and the other electrode on the other side the rotating disc structure with the associated apertures in the partition being located between the electrodes of the condensers, i. e. in the dielectric spaces thereof. The partition PSB is, of course, earthed and the disc CD is also earthed.

Figure 10 shows a preferred arrangement for coupling the aerial to the first stage of a high frequency amplifier or" a receiver in accordance with this invention. It has been found that with short wave working, considerable practical diihculty exists in coupling an ordinary earthed screened feeder leading from an antenna to the input circuit of a push-pull high frequency amplifier and the accompanying Figure 10 shows an arrangement in which this is employed in a satisfactory manner. Referring to Figure 10, the ends of the feeder wires (which ends are indicated by arrow heads) are tapped upon two coils 1L1 and IL2 which are connected in parallel with one another and with a condenser IC.

These coils and condensers are mounted as shown within a screening box 183 having a partition IP within which are formed windows or apertures IW. The coils 1L1 and IL2 are arranged opposite coils 1L3 and IL4, and the coils 11-1 and 1L2 1L3 and IL4 being so positioned that coil IL1 is magnetically coupled to coil 1L3 through a window IW, the coil 1L2 being also coupled to the coil ILi through a window IWQ In order to prevent electrostatic coupling, suitable wire is preferably stretched across the windows IW. Means for varying the magnetic coupling without necessitating movement of the coils are provided. 'A rotating disc and aperture structure similar to that shown in the accompanying Figure 9may be employed. The coils ILi and 1L2 and the condenser IC are earthed to the box 1813 at one end, and the coils 1L3 and IL4 are also earthed at one end, the other ends of the last mentioned coils being connected across a coil L5 which is connected as shown between the grids of the first stage of the high frequency amplifier, the valves of which are indicated in Figure 10 at HV1 and I-IVz. These valves are preferably screened grid valves. The push pull effeet is obtained by reversing the direction of winding coils IL; and IL4 and preferably the inductance of the coils IL; and IL; is made considerably bigger than that of the coils 1L1 and IL2 in order that variation of the effective inductance of the coil L5 due to variation of the coupling via the windows IW may be as small as possible. The tapping upon coils 1L1 and I'in are chosen so that the impedance presented by the circuit IL1 ICILz to the feeder connected. thereto, matches the surge impedance of said feeder.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

l. A superheterodyne receiver suitable for use on very short wave lengths and including a fre quency changer arrangement comprisingtwo mod-- ulating valves whose grids are electrostatically coupled in parallel to a source of local oscillations, said grids being also electrostatically coupled in push-pull to an input signal circuit.

2. A superheterodyne receiver suitable for use on very short wave lengths and including a frequency changer arrangement comprising two modulating valves whose grids are electrostatically coupled in parallel to a source of local oscillations, an adjustable capacitance located between each of said grids and said source of local oscillations, said grids being also electrostatically coupled in push-pull to an input signal circuit.

3. An arrangement as defined in claim 1 and in which bias for each grid is independently adjustable.

4. An arrangement as defined in claim 1 in which the two grids are connected to one an- .other through a pair of condensers in series, the

common point of said condensers being'connected to a coupling condenser through which the local oscillations are applied.

5. An arrangement as defined in claim 1 in which the two grids are connected together through a pair of condensers in series, said condensers being constituted by a mechanically unitary structure comprising a common electrode and two other electrodes adjustably positioned with respect thereto.

6. Translating apparatus having, in combination, a detector circuit comprising two electron discharge devices having their corresponding control electrodes connected together through two variable condensers serially arranged between said electrodes, a connection including a capacity from a point intermediate said condensers to a local oscillator circuit, separate 1 sources of biasing potentials for said control electrodes, and individual resistances intermediate said sources and said electrodes, a signal input circuit, and means for electrostatically connecting said control electrodes to said signal input circuit.

'7. Translating apparatus suitable for short waves having, in combination, two modulating electron discharge devices whose control elec trodes are connected together by a pair of serially connected and diflerentially arranged condensers whereby increase of capacity of one condenser is accompanied by decrease in capacity of the other, a signal input circuit, electrostatic connections from said control electrodes to said signal input circuit, a connection from the midpoint of said differentially arranged condenser structure to a source of local oscillations, and biasing means for said control electrodes.

8. A superheterodyne receiver comprising two push-pull connected electron discharge devices each having anode, cathode and control electrodes, a connection consisting solely of ,two serially arranged variable condensers between the control electrodes of said devices, individual biasing means for each control electrode, a source of local oscillations and an electrostatic connection from said source to a point between said two variable condensers, a signal input circuit coupled to both of said control electrodes through additional variable condensers individual to each control electrode.

9. A superheterodyne receiver comprising two push-pull connected electron discharge devices each having anode, cathode and control electrodes, a connection consisting solely of two serially arranged variable condensers between the control electrodes of said devices, individual biasing means for each control electrode, a source or" local oscillations and an electrostatic connection from said source to a point between said two vari. able condensers, a signal input circuit coupled to both of said control electrodes through additional variable condensers individual to each control electrode, anda choke coil connected in series with each of said last condensers.

10. A super-heterodyne receiver suitable for use on very short wave lengths and including a frequency changer arrangement comprising two modulating valves whose grids are electrostatically coupled in parallel to a source of local oscillations, a pair of amplifier valves, the grids of said modulating, valves being coupled in push-pull to the output circuits of said amplifying valves, said frequency changer arrangement being enclosed within a screening box and said amplifying valves being enclosed within another screening box.

11. A superheterodyne receiver suitable for use on very short wave lengths and including a frequency changer arrangement comprising two modulating valves whose grids are electrostatically coupled in parallel to a source of local oscillations', a pair of multi-electrode amplifier valves whose anodes are connected together through a high frequency choke coil, connections from the grids of said modulator valves to the anodes of the amplifier valves, each connection including two serially connected capacity reactances, and a circuit comprising an inductance in parallel with a capacitance extending from the junction point of the two capacities of one connection to the junction point of the two capacities of the other connection.

GASTON ADELIN MATHIEU. 

